1. Field of the Invention
The present invention relates to a light printer using electro-optical devices such as PLZT devices as light modulation means and more particularly to a light printer including a light shutter arranged between a light source and a photoconductor which is comprised of recording electro-optical elements aligned in a row and a recording control means for applying driving voltages to respective electro-optical elements according to recording data.
2. Description of the Prior Art
The light shutter providing electro-optical elements such as PLZT devices controls quantities of lights transmitting through respective electro-optical elements by applying driving voltages to them according to data to be recorded. It is known with respect to the electro-optical device that it has a temperature dependance and a driving voltage which gives a maximum transmittance, namely a half wavelength voltage varies dependently on a temperature thereof. Accordingly, if an identical driving voltage is applied to record to an identical recording data at different temperatures, the density of record varies with the temperature at that time.
In order to solve this problem, there has been proposed a light printer capable of controlling quantities of transmission light so that an identical density of record may be obtained to an identical recording data which provides with a monitoring electro-optical shutter for measuring the intensity of a transmission light transmitted therethrough and controls individual driving voltage according to results obtained thereby (See, for example, Japanese Utility Model laid-open publication No. S 63-189834).
However, even in the light printer mentioned above an identical density is not necessarily obtained to an identical recording data when used for a long time. This seems to be caused by variation of the half wavelength voltage due to internal polarization of the electro-optical material, namely optical fatigue caused by repetition of application of the driving voltage in use for a long time. Further, the degree of fatigue is different among electro-optical devices since the total amount of exposure differs among them and, accordingly, the amount of variation in the half wavelength voltage differs according to the degree of fatigue.
Since only one monitoring electro-optical element is provided in the light printer mentioned above, it is impossible to know the degree of fatigue with respect to individual shutter elements and, accordingly, is impossible to control the driving voltages according to degrees of fatigue of individual electro-optical elements. Further, if there exist some electro-optical elements having optical fatigue different from that of the monitoring electro-optical shutter, the resultant record becomes irregular when the light shutter is controlled according to data obtained from the monitoring shutter.